Long-life conductivity sensor system and method for using same

ABSTRACT

A long-life conductivity sensor system and method that is embeddable or immersible in a medium. The conductivity sensor system includes at least a housing with an enclosing wall that defines an interior volume and that has at least one aperture through the wall; a pair of electrodes protruding through the aperture into a medium surrounding the sensor housing; and conductivity sensing electronics contained within the sensor housing interior volume and connected to the pair of electrodes. The conductivity sensing electronics include a galvanostat connected to the electrodes for inducing discrete constant current pulses between the electrodes creating a transient voltage signal between the electrodes; and a high-speed voltmeter/A-D Converter connected to the electrodes for measuring the transient voltage signal between the electrodes, the transient voltage signal being a function of the conductivity of the medium surrounding the sensor housing.

FIELD OF THE INVENTION

The present invention relates generally to monitoring the conductivityof a medium. More particularly, the present invention is directed to along-life conductivity sensor system and method for monitoring theconductivity of a medium, such as concrete, by embedding or immersing asensor within the medium.

BACKGROUND OF THE INVENTION

Long-term monitoring of conductivity is often important in mediums suchas concrete, soil and fluids. Steel is widely used in reinforcingconcrete in buildings, bridges and roads. The conductivity of concretecan be used as a measure of corrosion of the steel. The electricalconductivity of concrete is usually low; however if concrete iscontaminated with salt, then the conductivity of concrete will increase.And salt—more specifically, the chloride in salt—can cause thereinforcing steel to corrode. The steel reinforcement bars embedded inthe concrete inherently have anodes and cathodes on the same surface,and the concrete acts as an electrolyte. Together, the anodes, cathodesand the electrolyte behave much like a short-circuited battery. Thiselectrochemical process causes corrosion of the anodic areas of thesteel, leaving the cathodic areas intact. If the concrete has beencontaminated with chloride, the conductivity of the concrete willincrease, which will accentuate corrosion, and eventually destroy thesteel.

The conductivity of soil is a useful parameter in agriculture becauseconductivity correlates generally to soil grain size and texture: sandshave a low conductivity, and silts have a medium conductivity. Clay canhave a high or low conductivity, depending upon its mineral content.Conductivity in shale and bedrock can vary with season due to floodingand water entrapment. Soils that have moderate conductivity, are mediumtextured, and have medium water-holding properties are often the mostagriculturally productive. Furthermore, long-term soil conductivity datacan be used to estimate the corrosion rates of buried metal pipelinesand metal conduits. Such data can also provide an indication of leaksand plumes originating from storage tanks and waste management areas,and contamination/pollution in soil sub-surfaces.

Many scientific disciplines require accurate, long-term monitoring ofthe conductivity of fluids. Environmentalists generally use conductivitydata to determine seasonal changes in the salinity of lakes and oceans.Chemical engineers are able to use conductivity data to monitorcorrosion rates in large holding tanks and industrial equipment.

The conductivity data in all of the above examples are generallyobtained using one of two types of conductivity sensors. One type is theinductive sensor, and the other is the electrode sensor. Theinductive-type conductivity sensor uses a toroidal input transformer toinduce a voltage in an electrolyte medium. A toroidal output transformermeasures the induced current, which is a function of the conductivity ofthe medium. Because of the nature of transformers, this type ofconductivity sensor is relatively large and is less common than theelectrode-type conductivity sensor.

Electrode-type conductivity sensors are further divided into sensorswith two electrodes and sensors with four electrodes. In thetwo-electrode sensors, generally, a current (I) is induced between thetwo electrodes by applying a potential or voltage difference (from anexternal power source) between those electrodes. The voltage difference(ΔV) and the current are measured and recorded. The ratio of voltagedifference to current (ΔV/I) provides the resistance from which theconductivity (κ) is computed as follows:

κ=k(I/ΔV)  (Eq. 1)

where k is the cell constant with units of cm⁻¹ or m⁻¹, and κ isconductivity with units of ohms⁻¹ cm⁻¹, ohm⁻¹ m⁻¹, mho cm⁻¹, mho m⁻¹, Scm⁻¹ or S m⁻¹. The value of k is determined for each pair of sensorsusing a medium of known conductivity.

The four-electrode conductivity sensors employ a second pair ofelectrodes. The first pair of electrodes passes a constant currentbetween them and through the medium. The second pair of electrodesmeasures the voltage difference between two points in the medium throughwhich current is passing. For example, a constant current is appliedacross two outer electrodes, and the conductivity of the mediumsurrounding the electrodes is then calculated using the values of thevoltage drop across two inner electrodes, the applied current, and thecell constant.

Prior art electrode-type conductivity sensors are widely adopted forspot-checking conductivity values and for short-term, in-line, andin-situ measurements. Existing applications include various types ofindustrial and environmental monitoring. However, a disadvantage of theprior art conductivity sensors of both the inductive type and theelectrode type is that they have not been capable of long-term, in-situmonitoring across a wide range of conductivity values.

A prior art corrosion sensor intended for long-term, in-situ measurementin reinforced concrete is described in U.S. Pat. No. 5,895,843, issuedApr. 20, 1999, to Taylor et al. (the '843 patent). It measures changesin the resistance in a steel wire that is buried in the concrete but notconnected to the rebar steel reinforcement. The wire is the sensor:corrosive agents entering the concrete corrode the wire, thinning thewire and changing its resistance. This is an indirect method to infercorrosion of the rebar. Rather than monitoring the conditions includinghigh conductivity that will eventually result in rebar corrosion, themethod described in the '843 patent detects corrosion only after damageto the wire, and by inference, damage to the rebar has occurred.Remedial action to correct a corrosive environment is likely to be moreeffective if taken early. Such early action is possible only if anenvironment is monitored directly, rather than simply detectingafter-the-fact the deleterious results.

Finally, the voltage applied across the electrodes of an electrode-typeconductivity sensor can introduce errors in the conductivitymeasurement. The conductivity of a medium is based on ion mobility. Whena DC voltage is applied across the electrodes of a conductivity sensor,the ions near the electrodes are quickly depleted and the electrodesbecome polarized. Such polarization results in measurements that arehigher than the actual resistance between the electrodes. Techniquesusing AC voltage have been developed to overcome this problem (see, forexample, U.S. Pat. No. 4,751,466, issued Jun. 14, 1988, to Colvin etal.); however, such techniques employ complex AC waveforms that requiresophisticated electronic components, which add to the cost and size ofthe corresponding sensor systems.

Therefore, particularly in the above-described areas ofsteel-reinforced-concrete, soil, and fluid monitoring, a need exists fora long-life conductivity sensor that may be permanently installed in alocation, that reliably monitors conductivity changes over severalorders of magnitude over a period of months or years, and that providesearly warning of potential corrosion.

SUMMARY OF THE INVENTION

The present invention, among other things, presents a solution to thepreviously discussed disadvantages associated with prior artconductivity sensors.

It is an object of the present invention to provide a conductivitysensor system that may be embedded in solids such as concrete or soil,or immersed in fluids such as chemical reagents found in holding tanks,to monitor changes in conductivity over several orders of magnitude overlong periods of time, up to several years.

Another object of the present invention is to provide a conductivitysensor system that is compact in size.

Yet another object of the present invention is to provide a conductivitysensor system that is of relatively low cost such that numerousconductivity sensors may be used in a single project, for example,embedded in a reinforced concrete bridge, while not significantlyincreasing the overall cost of the project.

Yet another object of the present invention is to provide a conductivitysensor system that overcomes the electrode polarization problemsassociated with some prior art conductivity sensors.

These and other objects are achieved in the present invention in aconductivity sensor system having at least a housing with an enclosingwall that defines an interior volume and that has at least one aperturethrough the wall; a pair of electrodes in contact with a mediumsurrounding the sensor housing; and conductivity-sensing electronicscontained within the sensor housing interior volume and connectedthrough the aperture in the wall to the pair of electrodes. Theconductivity-sensing electronics include a galvanostat connected to theelectrodes for inducing discrete, constant current pulses between theelectrodes, creating a transient voltage signal between the electrodes;and a high-speed voltmeter/A-D converter connected to the electrodes formeasuring the transient voltage signal between the electrodes, thetransient voltage signal being a function of the conductivity of themedium surrounding the sensor housing.

Other objects and advantages of the invention will become more fullyapparent from the following, more detailed description and the appendeddrawings, which illustrate several embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a conductivity sensor systemaccording to the present invention;

FIGS. 2A-2D show examples of several electrode designs according to thepresent invention; and

FIG. 3 illustrates a voltage transient between two electrodes.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic representation of a conductivity sensor systemaccording to the present invention. The conductivity sensor system ofthe present invention includes at least housing 12 having at least twoelectrodes 14 operatively connected to the conductivity-sensingcircuitry 16 enclosed within housing 12 and having at least agalvanostat 38 (a constant-current source) and a high-speedvoltmeter/A-D converter 20. In general, the housing 12 will be formedfrom conventional materials known in the art. Suitable materials for useherein include, but are not limited to, ceramic materials, for example,alumina and macor; plastic materials; nylon; concrete; and epoxy and thelike. As one skilled in the art would readily appreciate, dimensions andconfigurations for the housing 12 can vary as dictated by theapplication and can be determined on a case-by-case basis. However,employing a basic two-electrode design enables the size of the sensorhousing 12 to be relatively compact.

The sensor electronics are arranged inside the housing 12 and thehousing is sealed, for example, with chemically resistant epoxy. Theelectrodes 14 include typically a 10- to 100-micron-thick deposit ofgold on alumina; however platinum, nickel, or other highly conductivematerials may replace the gold. The electrodes are electricallyconnected to the electronics inside the housing 12 through an aperturein the wall of the housing 12.

A constant current is supplied across the electrodes 14 of theconductivity cell by the galvanostat 38. In one embodiment of thepresent invention, the galvanostat 38 consists of an amplifier 18, aresistor 30, a relay 28 and a power source 26 wired together as shown inFIG. 1. Relay 28 and resistor 30 provide overcurrent protection for thegalvanostat 38, as known in the art. The high-speed voltmeter/A-Dconverter 20 is connected across the conductivity cell to measure theresulting potential between the electrodes 14. The voltage data are thenoutputted via a data link 22 to a processor 24, for example, a processorwithin a laptop computer.

The sensor circuitry and data link 22 are also powered by the powersource 26 that may include a rechargeable nickel-cadmium or lithium-ionbattery or a super capacitor. It is contemplated that the power source26 can be internal to the sensor housing 12 when the sensor is embeddedin concrete or soil, and external to the sensor housing 12 when thesensor is immersed in a fluid inside a chemical container or a tank. Thepower source 26 is linked to an external power source such as a batteryfor recharging the power source 26. The link can be either through aninductive coupling 36 or direct wire contacts.

The relay 28 may also be used to place optional shunt resistors 32 inparallel with the relay circuit. A combination of shunt resistors 32 maybe selected to change the magnitude of the discrete constant-currentoutputs of the galvanostat 38. As described in the examples that follow,the ability to change the magnitude of the current output of thegalvanostat 38 greatly increases the range of any given sensor designhaving a fixed conductivity cell constant.

Temperature can also vary the conductivity of the ambient medium by asmuch as 1-3% per degree C. Therefore the present invention may includean optional temperature probe 34 as part of the conductivity sensorcircuitry. The voltage data from the temperature probe 34 is alsotransmitted through the data link 22.

The operator can determine the conductivity of the medium using themeasured voltage across the electrodes 14, the applied current, the cellconstant, and the measured temperature of the medium. Such calculationscan also be automated in the processor 24. As described in the examplesthat follow, the actual level of voltage depends upon the details of thegalvanostat 38 and the cell constant of the conductivity cell. Byadjusting the distance between the electrodes 14, their shape, and theirsize, any cell constant can be produced. By varying the current, thesensor can be used in various mediums with different initialconductivity values.

The present invention minimizes the errors found in many prior artsensors caused by depletion of ions near the electrodes 14 and theresulting polarization of the electrodes 14. As described previously,these errors result in measurements that are higher than the actualresistance. The errors are minimized by employing a technique known aschronopotentiometry. The galvanostat 38 applies a very short, discretecurrent pulse, I, across the two electrodes 14. A typical duration ofthe pulse is less than 4 milliseconds, and its magnitude varies from 0to I to 0. Because the pulse width is so short, ion depletion near theelectrodes 14 and the resulting errors from electrode polarization arenegligible. During and immediately after the pulse, the voltage acrossthe electrodes 14 varies with time. This voltage transient isillustrated in FIG. 3. A high-speed voltmeter/A-D converter 20 measuresthe difference between the maximum voltage, which occurs when the pulseis applied, and the minimum voltage, which is present after thetransient is fully attenuated. This ΔV is used to determine theconductivity using Eq. 1.

The following non-limiting examples are illustrative of the method formonitoring the conductivity of a medium over several orders of magnitudeemploying a conductivity sensor system in accordance with the presentinvention. In these examples, no physical access to or recalibration ofthe sensors is required.

EXAMPLES

Consider an embodiment of the present invention having avoltage-measuring device including an amplifier and a 12-bit A/Dconverter that operate between +3 and −3 V. The resulting maximumreadable voltage is ±0.0015 V, and the resolution is 3 mV. Duringexperimental measurements using this embodiment, the conductivity cellshaving the best performance consisted of two ring electrodes (14) havingthe following specifications: distance between the centers of the tworings=7.5 mm; ring OD=2.0 mm; ring ID=1.25 mm. FIGS. 2A-2D illustratethis embodiment and several other electrode designs according to thepresent invention. Now consider these limitations in conjunction withthe formula used in computing conductivity (κ)

κ=k(I/ΔV).  (Eq. 1)

Next consider a medium whose conductivity is assumed to change from0.001 S cm⁻¹ to 0.1 S cm⁻¹; the corresponding resistivity changes from1,000 to 10 ohm.cm. Such values can be expected in fresh water that iscontaminated by saline water. In this case, a current of 375 microampere(μA) is applied across a cell with a cell constant (κ) of 8 cm⁻¹. Thecombination of 375-μA current and 8 cm⁻¹ cell constant will generate 3 Vand 30 mV at 1,000 and 10 ohm.cm, respectively. These voltages arewithin the +3 V range and the 0.0015 V sensitivity of the 12-bit A/Dconverter.

Next consider the case of concrete, where the conductivity values couldchange from 3.3×10⁻⁶ to 3.3×10⁻⁴ S cm⁻¹. The corresponding resistivitychanges from 300,000 to 3,000 ohm.cm. In this case, a current of 10microampere is applied across a cell with a cell constant of 1 cm⁻¹. Thecombination of 10-μA current and 1 cm⁻¹ cell constant will generate 3 Vand 30 mV at 300,000 and 3,000 ohm.cm, respectively. These voltages areagain within the ±3 V range and the 3 mV sensitivity of the 12-bit A/Dconverter. In this fashion, embodiments of an embedded conductivitysensor according to the present invention may measure large changes inconductivity over several orders of magnitude without any need to accessthe embedded sensor physically.

In summary, the present invention provides for a long-life conductivitysensor system that is embeddable in solids such as concrete or soil, orimmersible in fluids such as chemical reagents found in holding tanks,and that can reliably monitor conductivity changes in these environmentsover a period of months or years. Also, the sensors are compact and havea relatively low cost such that numerous sensors may be used in a singleproject, for example embedded in a reinforced concrete bridge, while notsignificantly increasing the overall cost of the project. Furthermore,different embodiments of the present invention are adaptable toconductivity monitoring in diverse mediums such as concrete, soil, andfluids and can measure large changes in conductivity over several ordersof magnitude. While the above description contains many specifics, thereader should not construe these as limitations on the scope of theinvention, but merely as examples of specific embodiments thereof. Thoseskilled in the art will envision many other possible variations that arewithin its scope. Accordingly, the reader is requested to determine thescope of the invention by the appended claims and their legalequivalents, and not by the specific embodiments given above.

What is claimed is:
 1. A conductivity sensor system comprising: a sensorhousing having an enclosing wall that defines an interior volume andthat has an aperture through said wall; a pair of electrodes in contactwith a medium surrounding said sensor housing; and conductivity sensingelectronics contained within said sensor housing interior volume andoperatively connected through said aperture to said pair of electrodes,said conductivity sensing electronics comprising: a galvanostatoperatively connected to said electrodes for inducing a discreteconstant current pulses between said electrodes creating a transientvoltage signal between said electrodes; and a high-speed voltmeter/A-Dconverter operatively connected to said electrodes for measuring,between said electrodes, a difference between (i) a maximum of thetransient voltage signal occurring during the induced current pulse, and(ii) a minimum voltage occurring after the induced current pulse whenthe transient voltage signal is fully attenuated, the transient voltagesignal being a function of the conductivity of the medium surroundingsaid sensor housing.
 2. The conductivity sensor system of claim 1wherein the housing is a material selected from the group consisting ofceramic material, plastic material, nylon, concrete, epoxy andcombinations thereof.
 3. The conductivity sensor system of claim 1wherein said galvanostat induces discrete constant current pulses havinga duration less than 4 milliseconds.
 4. The conductivity sensor systemof claim 1 wherein said galvanostat induces discrete constant currentpulses having different magnitudes.
 5. The conductivity sensor system ofclaim 1 further comprising a temperature probe contained within saidsensor housing and operatively connected to said conductivity sensingelectronics for determining the temperature of the medium surroundingsaid sensor housing.
 6. The conductivity sensor system of claim 1wherein the conductivity sensing circuitry is powered by a power source.7. The conductivity sensor system of claim 6 wherein the power source isinside or outside said sensor housing.
 8. The conductivity sensor systemof claim 6 wherein the power source is a battery or capacitor.
 9. Amethod for monitoring the conductivity of a medium comprising the stepsof: providing a conductivity sensor system comprising a sensor housinghaving an enclosing wall that defines an interior volume and that has anaperture through said wall; a pair of electrodes protruding through saidaperture into a medium surrounding said sensor housing; and conductivitysensing electronics contained within said sensor housing interior volumeand operatively connected to said pair of electrodes, said conductivitysensing electronics comprising: a galvanostat operatively connected tosaid electrodes for inducing a discrete constant current pulses betweensaid electrodes creating a transient voltage signal between saidelectrodes; and a high-speed voltmeter/A-D Converter operativelyconnected to said electrodes for measuring, between said electrodes, adifference between (i) a maximum of the transient voltage signaloccurring during the induced current pulse, and (ii) a minimum voltageoccurring after the induced current pulse when the transient voltagesignal is fully attenuated, said transient voltage signal being afunction of the conductivity of the medium surrounding said sensorhousing; and, embedding said conductivity sensor system within themedium to be monitored.
 10. The method of claim 9 wherein the medium isselected from the group consisting of concrete, soil, storage tankscontaining chemical reagents or biological mediums.
 11. The method ofclaim 9 wherein said sensor housing of the conductivity sensor system isa material selected from the group consisting of ceramic material,plastic material, nylon, concrete, epoxy and combinations thereof. 12.The method of claim 9 wherein said galvanostat induces discrete constantcurrent pulses having a duration less than 4 milliseconds.
 13. Themethod of claim 9 wherein said galvanostat induces discrete constantcurrent pulses having different magnitudes.
 14. The method of claim 9wherein said conductivity sensor system further comprises a temperatureprobe contained within said sensor housing and operatively connected tosaid conductivity sensing electronics for determining the temperature ofthe medium surrounding said sensor housing.
 15. The method of claim 9wherein said conductivity sensing circuitry is powered by a powersource.
 16. The method of claim 9 wherein said power source is inside oroutside said sensor housing.
 17. The method of claim 9 wherein saidpower source is a battery or capacitor.
 18. A conductivity sensor systemcomprising: a sensor housing having an enclosing wall that defines aninterior volume and that has an aperture through said wall; a pair ofelectrodes protruding through said aperture into a medium surroundingsaid sensor housing; current inducing means for inducing a discreteconstant current pulse between said electrodes, said current inducingmeans contained within said sensor housing interior volume andoperatively connected to said electrodes; and high-speed voltagemeasuring means for measuring, between said electrodes, a differencebetween (i) a maximum of the transient voltage signal occurring duringthe induced current pulse, and (ii) a minimum voltage occurring afterthe induced current pulse when the transient voltage signal is fullyattenuated, said high-speed voltage measuring means contained withinsaid sensor housing interior volume and operatively connected to saidelectrodes, the transient voltage signal being a function of theconductivity of the medium surrounding said sensor housing.
 19. Theconductivity sensor system of claim 18 wherein the housing is a materialselected from the group consisting of ceramic material, plasticmaterial, nylon, concrete, epoxy and combinations thereof.
 20. Theconductivity sensor system of claim 18 wherein said galvanostat inducesdiscrete constant current pulses having a duration less than 4milliseconds.
 21. The conductivity sensor system of claim 18 whereinsaid galvanostat induces discrete constant current pulses havingdifferent magnitudes.
 22. The conductivity sensor system of claim 18further comprising a thermocouple probe contained within said sensorhousing and operatively connected to said conductivity sensingelectronics for determining the temperature of the medium surroundingsaid sensor housing.
 23. The conductivity sensor system of claim 18wherein the conductivity sensing circuitry is powered by a power source.24. The conductivity sensor system of claim 18 wherein the power sourceis inside or outside said sensor housing.
 25. The conductivity sensorsystem of claim 18 wherein the power source is a battery or capacitor.